/Engine/Source/Runtime/SlateCore/Public/Widgets/SPanel.h
Each child widget should be stored in a Slot. The Slot describes how the individual child should be arranged with respect to its parent (i.e. the Panel) and its peers Widgets (i.e. the Panel's other children.) For a simple example see StackPanel.
class SPanel : public [SWidget](API\Runtime\SlateCore\Widgets\SWidget)